1. Field of Invention
This invention relates to the field of tungsten carbide materials suitable for cutting and wear applications.
2. Description of the Prior Art of the Invention
Cemented tungsten carbide materials have become the working horse for a large number of machining and wear applications. These materials exhibit a Co binder phase which enables sintering at relative low temperatures while the ductile phase provides a relative high toughness and transverse rupture strength. On the other side, the ductile Co binder reduces hardness and temperature stability. However, during the last decades performance requirements of carbide cutting tools have become ever more demanding due to increased cutting speeds and new work materials (super-alloys, titanium alloys, metal matrix composites, etc) requiring higher contact point temperatures and wear resistance. For hardened steels and nickel based super alloys, cutting tools must have high wear resistance at the cutting edge. At high cutting speeds the temperature at the cutting tip reaches temperatures around 800 to 1000 xc2x0 C., demanding excellent high-temperature strength and wear resistance of the tool material without reacting with the work piece. Diffusion processes in the tool material, which can be promoted by metal impurities and residual metal binders phases, can potentially increase the wear and must be kept as low as possible. The machining of a relatively brittle alloy like cast iron creates may cause chipping of the cutting tool edge which demands high toughness and hardness of the tool bit. In case of machining aluminum, the wear is mostly adhesive. The chip may stick on the cutting edge decreasing the quality of the machined surface. The tool material therefore needs to have a low friction with the work piece and good thermal conductivity to decrease the temperature at the cutting point. The particular problem of machining titanium alloys is their high toughness causing significant wearxe2x80x94in particular crater wearxe2x80x94on the tool. This is enhanced by the strong reactivity of Ti with many cutting tool materials. In particular the binder phase in cemented carbides promotes the reaction with Ti.
Consequently, various attempts have been made to reduce or omit the binder content in polycrystalline tungsten carbide.
However, the lower the binder content becomes the higher sintering temperatures and pressures have to be in order to produce dense compacts. The higher temperatures required for sintering conventional submicron WC typically result in excessive grain growth which results in a brittle material with only moderate performance in machining applications. The literature suggests a number of routes to manufacture such a dense body. It is then possible to use nonstoichiometric tungsten carbide which exhibits a higher reactivity during sintering and can be consolidated with reasonable effort. However, the non-stoichiometry enhances grain growth during sintering which is undesirable for cutting tool applications. In a similar way mixtures of W, WC, carbon, and other carbides and nitrides promote reactive sintering which accelerates densification. An unwelcome result is that the content of phases softer than WC increases thus limiting the overall hardness and wear resistance in particular at high temperatures. Additional phases also mean a higher chemical reactivity with the work piece material and decreasing wear resistance of the tool bit which becomes critical at temperatures around 1000 xc2x0 C. which occur under modern high-high-speed machining conditions are required for modern machining conditions.
A further approach is to mill binderless tungsten carbide with cemented tungsten carbide balls. The latter produce a well distributed contamination of cobalt in the powder. While the cobalt contamination improves the sinterability of the WC powder, grain growth also can become a problem thus reducing the materials fracture toughness. Consequently, strength and hardness are below the desirable optimum. In fact, all procedures described to make a xe2x80x9cbinderlessxe2x80x9d tungsten carbide contain Co at least on a contamination level which is more than 0.1% by weight.
All methods utilizing more or less binderless powders or powder mixtures require pressure (hot-pressing) and relative high temperatures, typically 200-500 xc2x0 C. higher than cemented carbide. The most common techniques are hot-pressing and hot isostatic pressing (HIP). If the sintering technique allows for high pressures like HIPing, the temperature can be reduced which enables to maintain a small grain size in the sintered body. HIPing, however, is a complicated process which requires encapsulation of the green compact or powder batch. The encapsulation process usually seals the specimen off which traps impurities or surface oxides which are undesirable. Detailed analysis of commonly produced xe2x80x9cbinderlessxe2x80x9d tungsten carbide show that always small quantities of residual phases in between the WC grains exist. This can only be overcome by an additional costly reduction treatment. Conventional hot-pressing or similar techniques like xe2x80x9cRapid Omn-idirectional Compactionxe2x80x9d also do not address the purification need adequately and are commonly slow processes thus enhancing the grain growth problem at higher temperatures. Contaminating impurities can diffuse from the furnace environment into the specimen, and the surface oxides on the powder particles are usually not significantly reduced during sintering thus retarding the densification and reducing the strength of the sintered body.
The patent literature also describes the application of xe2x80x9cbinderlessxe2x80x9d tungsten carbide for machining titanium. While titanium is definitely a critical application it would be desirable to have a more versatile tungsten carbide which is suitable for a wider range of machining applications and tool materials.
A need therefore exists to produce a high-purity, stoichiometric tungsten carbide material with ultrafine grain size in order to obtain a very strong, tough and hard body with excellent high-temperature chemical and mechanical stability and good thermal conductivity.
A further need exists for a sintering process which is fast, economical, and able to remove surface oxidesxe2x80x94in particular of nanosized powdersxe2x80x94in an efficient way.
Disclosed is a new polycrystalline tungsten carbide material, its manufacturing and applications for tools like cutting inserts used in turning, milling, honing and drilling of a wide range of metals, plastics, ceramics and wood, and highly wear resistant parts like wire-drawing dies. The material is made from an ultrafine, very pure tungsten carbide powder without any metal binder additions and sintered with a special, purifying technique into a dense, strong, hard polycrystalline body. Cutting tools or inserts made from this material are particularly suited for high-speed machining, hard turning and rough turning of steels and ferrous alloys, alloys which are difficult to be machined due to work hardening, like nickel superalloys, and titanium and its alloys. The performance of the disclosed material is almost comparable to the performance of polycrystalline cubic boron nitride (cBN), without requiring the expensive ultra-high pressure process of manufacturing cBN, and substantially superior to binderless WC cutting tool materials reported to date.